Onboard refueling vapor recovery (ORVR) systems for controlling the flow of fuel vapor from a vehicle fuel tank to a recovery apparatus such as a carbon canister are generally known. A common approach is to place a control valve in series between the fuel tank and the canister to selectively open and close the vapor venting pathway in response to changes in vehicle refueling activity.
Some known ORVR control valves use a diaphragm which opens during refueling so that the fuel vapor displaced by the introduction of liquid fuel into the tank is allowed to vent to the vapor canister. An example of a diaphragm-operated valve is shown in U.S. Pat. No. 4,714,172 to Morris.
Filler nozzles used at many commercial filling stations have diaphragm-operated shutoff mechanisms which require a supply of make-up air or vapor to continue delivering fuel, and which automatically shut off when the make-up air supply is cut off. Make-up air is typically supplied by a nozzle shutoff line extending into the fuel tank to the full fuel level, and shutoff occurs when the shutoff line is covered by liquid fuel when the tank is full.
If the fuel in the tank is hot when refueling begins, the introduction of cold fuel from an offboard supply causes the hot fuel vapor in the tank to contract as it cools. This contraction can create vacuum conditions inside the tank which tend to interfere with the operation of pressure-operated diaphragm valves. Certain types of diaphragm-operated ORVR valves may prematurely close during refueling in response to these vacuum conditions. Diaphragm shutoff mechanisms in the filler nozzle can also be affected to prematurely close, interrupting the refueling operation.
One solution for preventing vacuum induced premature shutoff involves providing a vacuum relief bypass somewhere in the ORVR system. This, however, can introduce added complexity with attendant increases in both cost and likelihood of valve failure modes.